The invention relates to a fluoride-sensitive electrode and the use of said electrode in measuring the fluoride content of water, in particular in fluoridated drinking water.
In many countries of the world, fluoride is added to drinking water in an amount of about 0.5 to 2 ppm to protect against caries. The fluoride content of drinking water is monitored using fluoride-sensitive electrodes by a potentiometric measurement, as disclosed in 1969 in U.S. Pat. No. 3,431,182. Such electrodes employ lanthanum fluoride as the ion-sensitive element and operate with an internal fluoride-containing electrolyte. The fluoride-sensitive electrode constitutes an electrochemical half-cell which is combined with a conventional reference electrode to determine a potential. If the reference electrode is integrated into the housing of the fluoride-sensitive electrode, it is termed a combination electrode.
Despite their excellent selectivity for fluoride ions, known fluoride-sensitive electrodes exhibit a disturbing cross-sensitivity to hydroxyl ions (OH ions) which increases towards lower fluoride contents. The pH of drinking water or sample water, the fluoride concentration of which is to be determined with the electrode of the invention, will henceforth be termed the “external pH” or “pHa”. Since the pHa of drinking water can lie in the range 6.5 to 9.5 and it is often raised by the supplier to slightly alkaline values in order to make the water less corrosive, for example, this cross-sensitivity to hydroxyl ions disturbs the accuracy of the fluoride-sensitive measuring electrode beyond a pHa of about 8.5. To overcome this and other cross-sensitivities, TISAB (total ionic strength adjustment buffer) has been used since 1968. The sample water is mixed with TISAB solution to reduce the pHa of the solution to be measured to about 5.5, inter alia, and thus avoid hydroxyl ion cross-sensitivity. This conditioning of the sample water with TISAB constitutes an additional test costing and in particular, continuous measurement is rendered uneconomical because of the considerable cost of TISAB. Commercially available automated fluoride ion analyzers take samples at selected intervals, mix them with TISAB and then determine the fluoride ion content. Automation using such devices, though, does not avoid the high cost of TISAB.
Thus, the aim of the present invention is to provide a fluoride-sensitive electrode with a lower cross-sensitivity to hydroxyl ions than known electrodes and which also enables accurate measurements of the fluoride content of water with higher pHa values, in particular at low fluoride concentrations, to be carried out without requiring the use of TISAB.